Percentile from value
Closes #16 (closed).
Functionality and examples to support the interpolation of the percentile values using previously calculated percentile thresholds and values.
Adds function calculate_percentile_from_value
and associated unit tests. Also adds an example documentation page walking through the use of all the percentile calculation functions and explaining what they do and why. Doc-string examples in the API reference use synthetic data while these new examples demonstrate the fetching and analysis of streamflow data from NWIS.
Note that this function currently only does linear interpolation of values. Extrapolation is not supported, and so values below previously established 0th percentile return a 0 percentile, and values above the previously established 100th percentile will return a 100.